The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a cellular phone, a portable information terminal, or the like, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, i.e., multifunctional cellular phones which can run various application software as well as a voice call function, have been more widely used. When application software is run on smartphones, it is possible to perform functions such as those of digital still cameras and car navigation systems on the smartphones. In order to perform those various functions, most models of smartphones include cameras similar to the cellular phones.
Generally speaking, product groups of such smartphones are often composed according to specifications for beginners to advanced users. Among them, an imaging lens to be mounted in a product designed for the advanced users is required to have a high-resolution lens configuration so as to be also applicable to a high pixel count imaging element of these years.
As one of methods of attaining the high-resolution imaging lens, there has been a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens. Therefore, the lens configuration having a large number of lenses has a disadvantage in terms of mounting in a small-sized camera such as the above-described smartphones. For this reason, conventional imaging lenses have been developed so as to reduce the number of lenses therein as much as possible. However, with rapid advancement in achieving the higher pixel count of an imaging element in these days, an imaging lens has been developed so as to attain higher resolution rather than a shorter total track length of the imaging lens. For example, while it has been conventionally common to mount a camera unit, which includes an imaging lens and an imaging element, inside a smartphone, in these years, there has also been an attempt to attach a separate camera unit onto a smartphone, whereby it is possible to obtain images equivalent to those of digital still cameras.
In case of a lens configuration composed of seven lenses, due to the large number of lenses of the imaging lens, although it is slightly disadvantageous for downsizing of the imaging lens, it has high flexibility in design. In addition, it has potential to attain satisfactory correction of aberrations, and downsizing of the imaging lens in a balanced manner. For example, as the imaging lens having the seven-lens configuration as described above, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2012-155223
The conventional imaging lens described in Patent Reference includes a first lens that has a shape of a biconvex shape, a second lens that has a shape of a biconcave shape joined to the first lens, a third lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to the object side, a fourth lens that is positive and has a shape of a meniscus lens directing a concave surface thereof to the object side, a fifth lens that is negative and directs a convex surface thereof to the object side, a sixth lens that has a biconvex shape, and a seventh lens that has a biconcave shape, arranged in the order from the object side. According to the conventional imaging lens of Patent Reference, the first through the fourth lenses compose a first lens group and the fifth through the seventh lenses compose a second lens group. With the configuration, it is designed to restrain a ratio of a focal length of the first lens group relative to that of the second lens group within a certain range, so that it is achievable to downsize the imaging lens and satisfactorily correct aberrations.
In case of the conventional imaging lens of Patent Reference, although the size of the imaging lens is small, correction of the image surface is insufficient and the distortion is especially large. Therefore, there is a limit by itself to achieve a high performance imaging lens. With the lens configuration of the imaging lens of Patent Reference, it is difficult to achieve satisfactory aberration correction while downsizing of the imaging lens.
It should be noted that such a problem is not specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is a common problem for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems of the conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing thereof and satisfactory aberration correction.
Further objects and advantages of the present invention will be apparent from the following description of the invention.